Tuesday, November 29, 2011

PHP is an open-source server-side scripting language and it is a widely used. The Apache web server provides access to files and content via the HTTP OR HTTPS protocol. A misconfigured server-side scripting language can create all sorts of problems. So, PHP should be used with caution. Here are twenty-fivephp security best practices for sysadmins for configuring PHP securely.

Our Sample Setup For PHP Security Tips

DocumentRoot: /var/www/html

Default Web server: Apache ( you can use Lighttpd or Nginx instead of Apache)

Operating systems: RHEL / CentOS / Fedora Linux (the instructions should work with any other Linux distributions such as Debian / Ubuntu or other Unix like operating systems such as OpenBSD/FreeBSD/HP-UX).

Default php server TCP/UDP ports: none

Most of the actions listed in this post are written with the assumption that they will be executed by the root user running the bash or any other modern shell:$ php -v
Sample outputs:

For demonstration purpose I'm going to use the following operating system:$ cat /etc/redhat-release
Sample outputs:

Red Hat Enterprise Linux Server release 6.1 (Santiago)

#1: Know Your Enemy

PHP based apps can face the different types of attacks. I have noticed the different types of attacks:

XSS - Cross-site scripting is a vulnerability in php web applications, which attackers may exploit to steal users' information. You can configure Apache and write more secure PHP scripts (validating all user input) to avoid xss attacks.

SQL injection - It is a vulnerability in the database layer of an php application. When user input is incorrectly filtered any SQL statements can be executed by the application. You can configure Apache and write secure code (validating and escaping all user input) to avoid SQL injection attacks. A common practice in PHP is to escape parameters using the function called mysql_real_escape_string() before sending the SQL query.Spoofing

File uploads - It allows your visitor to place files (upload files) on your server. This can result into various security problems such as delete your files, delete database, get user details and much more. You can disable file uploads using php or write secure code (like validating user input and only allow image file type such as png or gif).

Including local and remote files - An attacker can open files from remote server and execute any PHP code. This allows them to upload file, delete file and install backdoors. You can configure php to disable remote file execution.

eval() - Evaluate a string as PHP code. This is often used by an attacker to hide their code and tools on the server itself. You can configure php to disable eval().

Sea-surf Attack (Cross-site request forgery - CSRF) - This attack forces an end user to execute unwanted actions on a web application in which he/she is currently authenticated. A successful CSRF exploit can compromise end user data and operation in case of normal user. If the targeted end user is the administrator account, this can compromise the entire web application.

#2: Find Built-in PHP Modules

To see the set of compiled-in PHP modules type the following command:# php -m
Sample outputs:

I recommends that you use PHP with a reduced modules for performance and security. For example, you can disable sqlite3 module by deleting (removing) configuration file , ORrenaming (moving) a file called /etc/php.d/sqlite3.ini as follows:# rm /etc/php.d/sqlite3.ini
OR# mv /etc/php.d/sqlite3.ini /etc/php.d/sqlite3.disable
Other compiled-in modules can only be removed by reinstallating PHP with a reduced configuration. You can download php source code from php.net and compile it as follows with GD, fastcgi, and MySQL support:

#3: Restrict PHP Information Leakage

To restrict PHP information leakage disable expose_php. Edit /etc/php.d/secutity.ini and set the following directive:

expose_php=Off

When enabled, expose_php reports to the world that PHP is installed on the server, which includes the PHP version within the HTTP header (e.g., X-Powered-By: PHP/5.3.3). The PHP logo guids (see example) are also exposed, thus appending them to the URL of a PHP enabled site will display the appropriate logo. When expose_php enabled you can see php version using the following command:$ curl -I http://www.cyberciti.biz/index.php
Sample outputs:

#4: Minimize Loadable PHP Modules (Dynamic Extensions)

PHP supports "Dynamic Extensions". By default, RHEL loads all the extension modules found in /etc/php.d/ directory. To enable or disable a particular module, just find the configuration file in /etc/php.d/ directory and comment the module name. You can also rename or delete module configuration file. For best PHP performance and security, you should only enable the extensions your webapps requires. For example, to disable gd extension, type the following commands:# cd /etc/php.d/# mv gd.{ini,disable}# /sbin/service httpd restart
To enable php module called gd, enter:# mv gd.{disable,ini}# /sbin/service httpd restart

#5: Log All PHP Errors

Do not expose PHP error messages to all site visitors. Edit /etc/php.d/security.ini and set the following directive:

#7: Turn Off Remote Code Execution

If enabled, allow_url_fopen allows PHP's file functions -- such as file_get_contents() and the include and require statements -- can retrieve data from remote locations, like an FTP or web site.

The allow_url_fopen option allows PHP's file functions - such as file_get_contents() and the include and require statements - can retrieve data from remote locations using ftp or http protocols. Programmers frequently forget this and don't do proper input filtering when passing user-provided data to these functions, opening them up to code injection vulnerabilities. A large number of code injection vulnerabilities reported in PHP-based web applications are caused by the combination of enabling allow_url_fopen and bad input filtering. Edit /etc/php.d/security.ini and set the following directive:

allow_url_fopen=Off

I also recommend to disable allow_url_include for security reasons:

allow_url_include=Off

#8: Enable SQL Safe Mode

Edit /etc/php.d/security.ini and set the following directive:

sql.safe_mode=On

If turned On, mysql_connect() and mysql_pconnect() ignore any arguments passed to them. Please note that you may have to make some changes to your code. Third party and open source application such as WordPress, and others may not work at all when sql.safe_mode enabled. I also recommend that you turn off magic_quotes_gpc for all php 5.3.x installations as the filtering by it is ineffective and not very robust. mysql_escape_string() and custom filtering functions serve a better purpose (hat tip to Eric Hansen):

magic_quotes_gpc=Off

#9: Control POST Size

The HTTP POST request method is used when the client (browser or user) needs to send data to the Apache web server as part of the request, such as when uploading a file or submitting a completed form. Attackers may attempt to send oversized POST requests to eat your system resources. You can limit the maximum size POST request that PHP will process. Edit /etc/php.d/security.ini and set the following directive:

; Set a realistic value herepost_max_size=1K

The 1K sets max size of post data allowed by php apps. This setting also affects file upload. To upload large files, this value must be larger than upload_max_filesize. I also suggest that you limit available methods using Apache web server. Edit, httpd.conf and set the following directive for DocumentRoot /var/www/html:

Order allow,deny
## Add rest of the config goes here... ##

#10: Resource Control (DoS Control)

You can set maximum execution time of each php script, in seconds. Another recommend option is to set maximum amount of time each script may spend parsing request data, and maximum amount of memory a script may consume. Edit /etc/php.d/security.ini and set the following directives:

#11: Install Suhosin Advanced Protection System for PHP

Suhosin is an advanced protection system for PHP installations. It was designed to protect servers and users from known and unknown flaws in PHP applications and the PHP core. Suhosin comes in two independent parts, that can be used separately or in combination. The first part is a small patch against the PHP core, that implements a few low-level protections against bufferoverflows or format string vulnerabilities and the second part is a powerful PHP extension that implements all the other protections.

#13 PHP Fastcgi / CGI - cgi.force_redirect Directive

PHP work with FastCGI. Fascgi reduces the memory footprint of your web server, but still gives you the speed and power of the entire PHP language. You can configureApache2+PHP+FastCGI or cgi as described here. The configuration directive cgi.force_redirect prevents anyone from calling PHP directly with a URL like http://www.cyberciti.biz/cgi-bin/php/hackerdir/backdoor.php. Turn on cgi.force_redirect for security reasons. Edit /etc/php.d/security.ini and set the following directive:

#14 PHP User and Group ID

mod_fastcgi is a cgi-module for Apache web server. It can connect to an external FASTCGI server. You need to make sure php run as non-root user. If PHP executes as a root or UID under 100, it may access and/or manipulate system files. You must execute PHP CGIs as a non-privileged user using Apache's suEXEC or mod_suPHP. The suEXEC feature provides Apache users the ability to run CGI programs under user IDs different from the user ID of the calling web server. In this example, my php-cgi is running as phpcgi user and apache is running as apache user:# ps aux | grep php-cgi
Sample outputs:

#15 Limit PHP Access To File System

The open_basedir directive set the directories from which PHP is allowed to access files using functions like fopen(), and others. If a file is outside of the paths defined by open_basdir, PHP will refuse to open it. You cannot use a symbolic link as a workaround. For example only allow access to /var/www/html directory and not to /var/www, or /tmp or /etc directories:

#16 Session Path

Session support in PHP consists of a way to preserve certain data across subsequent accesses. This enables you to build more customized applications and increase the appeal of your web site. This path is defined in /etc/php.ini file and all data related to a particular session will be stored in a file in the directory specified by the session.save_path option. The default is as follows under RHEL/CentOS/Fedora Linux:

session.save_path="/var/lib/php/session"; Set the temporary directory used for storing files when doing file upload
upload_tmp_dir="/var/lib/php/session"

Make sure path is outside /var/www/html and not readable or writeable by any other system users:# ls -Z /var/lib/php/
Sample outputs:

Note: Check php.net for the most recent release for source code installations.

#18: Restrict File and Directory Access

Make sure you run Apache as a non-root user such as Apache or www. All files and directory should be owned by non-root user (or apache user) under /var/www/html:# chown -R apache:apache /var/www/html/
/var/www/html/ is a subdirectory and DocumentRoot which is modifiable by other users since root never executes any files out of there, and shouldn't be creating files in there.

A Note About Setting Up Correct File Permissions

The chown and chmod command make sures that under no circumstances DocumentRoot or files contained in DocumentRoot are writable by the Web server user apache. Please note that you need to set permissions that makes the most sense for the development model of your website, so feel free to adjust the chown and chmod command as per your requirements. In this example, the Apache server run as apache user. This is configured with the User and Groupdirectives in your httpd.conf file. The apache user needs to have read access to everything under DocumentRoot but should not have write access to anything.

Make sure httpd.conf has the following directives for restrictive configuration:

Options None
AllowOverride None
Order allow,deny

You should only grant write access when required. Some web applications such as wordpress and others may need a caching directory. You can grant a write access to caching directory using the following commands:# chmod a+w /var/www/html/blog/wp-content/cache### block access to all #### echo 'deny from all' > /var/www/html/blog/wp-content/cache/.htaccess

#20: Use Linux Security Extensions (such as SELinux)

Linux comes with various security patches which can be used to guard against misconfigured or compromised server programs. If possible use SELinux and other Linux security extensions to enforce limitations on network and other programs. For example, SELinux provides a variety of security policies for Linux kernel and Apache web server. To list all Apache SELinux protection variables, enter:# getsebool -a | grep httpd
Sample outputs:

allow_httpd_anon_write --> off
allow_httpd_mod_auth_ntlm_winbind --> off
allow_httpd_mod_auth_pam --> off
allow_httpd_sys_script_anon_write --> off
httpd_builtin_scripting --> on
httpd_can_check_spam --> off
httpd_can_network_connect --> off
httpd_can_network_connect_cobbler --> off
httpd_can_network_connect_db --> off
httpd_can_network_memcache --> off
httpd_can_network_relay --> off
httpd_can_sendmail --> off
httpd_dbus_avahi --> on
httpd_enable_cgi --> on
httpd_enable_ftp_server --> off
httpd_enable_homedirs --> off
httpd_execmem --> off
httpd_read_user_content --> off
httpd_setrlimit --> off
httpd_ssi_exec --> off
httpd_tmp_exec --> off
httpd_tty_comm --> on
httpd_unified --> on
httpd_use_cifs --> off
httpd_use_gpg --> off
httpd_use_nfs --> off

#23 Use Firewall To Restrict Outgoing Connections

The attacker will download file locally on your web-server using tools such as wget. Use iptables to block outgoing connections from apache user. The ipt_owner module attempts to match various characteristics of the packet creator, for locally generated packets. It is only valid in the OUTPUT chain. In this example, allow vivek user to connect outside using port 80 (useful for RHN or centos repo access):

#24 Watch Your Logs & Auditing

Check the apache log file:# tail -f /var/log/httpd/error_log# grep 'login.php' /var/log/httpd/error_log# egrep -i "denied|error|warn" /var/log/httpd/error_log
Check the php log file:# tail -f /var/log/httpd/php_scripts_error.log# grep "...etc/passwd" /var/log/httpd/php_scripts_error.log
Log files will give you some understanding of what attacks is thrown against the server and allow you to check if the necessary level of security is present or not. The auditd service is provided for system auditing. Turn it on to audit SELinux events, authetication events, file modifications, account modification and so on. I also recommend using standard "Linux System Monitoring Tools" for monitoring your web-server.

#25 Run Service Per System or VM Instance

For large installations it is recommended that you run, database, static, and dynamic content from different servers.

Run different network services on separate servers or VM instances. This limits the number of other services that can be compromised. For example, if an attacker able to successfully exploit a software such as Apache flow, he / she will get an access to entire server including other services running on the same server (such as MySQL, e-mail server and so on). But, in the above example content are served as follows:

static.lan.cyberciti.biz - Use lighttpd or nginx server for static assets such as js/css/images.

phpcgi1.lan.cyberciti.biz and phpcgi2.lan.cyberciti.biz - Apache web-server with php used for generating dynamic content.

mysql1.lan.cyberciti.biz - MySQL database server.

mcache1.lan.cyberciti.biz - Memcached server is very fast caching system for MySQL. It uses libevent or epoll (Linux runtime) to scale to any number of open connections and uses non-blocking network I/O.

LB01 - A nginx web and reverse proxy server in front of Apache Web servers. All connections coming from the Internet addressed to one of the Web servers are routed through the nginx proxy server, which may either deal with the request itself or pass the request wholly or partially to the main web servers. LB01 provides simple load-balancing.

#26 Additional Tools

PHPIDS (PHP-Intrusion Detection System) is a simple to use, well structured, fast and state-of-the-art security layer for your PHP based web application. The IDS neither strips, sanitizes nor filters any malicious input, it simply recognizes when an attacker tries to break your site and reacts in exactly the way you want it to.

You can use PHPIDS to detect malicious users, and log any attacks detected for later review. Please note that I've personally not used this tool.

PhpSecInfo provides an equivalent to the phpinfo() function that reports security information about the PHP environment, and offers suggestions for improvement. It is not a replacement for secure development techniques, and does not do any kind of code or app auditing, but can be a useful tool in a multilayered security approach.

A Note About PHP Backdoors

You may come across php scripts or so called common backdoors such as c99, c99madshell, r57 and so on. A backdoor php script is nothing but a hidden script for bypassing all authentication and access your server on demand. It is installed by an attackers to access your server while attempting to remain undetected. Typically a PHP (or any other CGI script) script by mistake allows inclusion of code exploiting vulnerabilities in the web browser. An attacker can use such exploiting vulnerabilities to upload backdoor shells which can give him or her a number of capabilities such as:

Conclusion

Your PHP based server is now properly harden and ready to show dynamic webpages. However, vulnerabilities are caused mostly by not following best practice programming rules. You should be consulted further resources for your web applications security needs especially php programming which is beyond the scope of sys admin work.

Recommended readings:

PHP Security Guide: This guide aims to familiarise you with some of the basic concepts of online security and teach you how to write more secure PHP scripts. It's aimed squarely at beginners, but I hope that it still has something to offer more advanced users.

Essential PHP Security (kindle edition): A book about web application security written specifically for PHP developers. It covers 30 of the most common and dangerous exploits as well as simple and effective safeguards that protect your PHP applications.

SQL Injection Attacks and Defense This book covers sql injection and web-related attacks. It explains SQL injection. How to find, confirm, and automate SQL injection discovery. It has tips and tricks for finding SQL injection within the code. You can create exploits using SQL injection and design to avoid the dangers of these attacks.